GS 4104 is a neuraminidase inhibitor with demonstrated anti-human influenza activity in vitro and in vivo. It has been administered to humans without producing serious adverse effects. This study is a multicenter, double-blind, randomized, placebo controlled study with three study groups. healthy adults, aged 18-65 years will be assigned to one of three treatment groups: 1)GS 4104 75 mg po qd; 2)GS 4104 75 mg po bid; and 3) matching placebo. Duration of prophylaxis is six weeks and will be initiated when the first cases of influenza are noted in the Galveston area. The specific aims are: 1) To investigate the efficacy of GS 4104 in preventing laboratory confirmed clinical influenza; 2) to investigate the efficacy of GS 4104 in preventing asymptomatic laboratory confirmed influenza infection; and 3) to investigate the safety and tolerability of GS 4104. The overarching hypothesis is that GS 4104 recipients will have less influenza disease than placebo recipients. The results obtained from this study may provide an alternative to influenza vaccination, amantadine and rimantadine for the prevention of influenza. An alternative to these drugs would be medically useful since some people cannot tolerate influenza vaccines due to allergy or other adverse effects, influenza vaccine provides incomplete prophylaxis since the vaccine provides protection against influenza species from the previous year's strains of influenza and the use of amantadine and rimantadine is limited by GI and CNS side effects as well as the development of viral resistance to these agents.